Orientifold limit of F-theory vacua

We show how an F-theory compactified on a Calabi-Yau (n+1)-fold in appropriate weak coupling limit reduces formally to an orientifold of type IIB theory compactified on an auxiliary complex n-fold. In some cases (but not always) if the original (n+1)- fold is singular, then the auxiliary n-fold is also singular. We illustrate this by analysing F-theory on elliptically fibered Calabi-Yau 3-folds on base Fn